This invention relates to a Maintenance Termination Unit (or MTU) for use, particularly but not exclusively, in telephone lines.
Typically, a Maintenance Termination Unit or an MTU is specified as having two Voltage Sensitive Switches, VSS1 and VSS2, one in each telephone line wire a and b, and a Distinctive Termination Unit connected between the line wires on the subscribers side (equipment side) of the MTU. A diagrammatical example of such an MTU is shown in FIG. 1 of the accompanying drawings.
Much of the functionality and many of the parameter values of the Voltage Sensitive Switches are dictated by industry accepted standard specifications. In terms of switching voltage there are maximum and minimum limits. Typically, the lower level of the switching voltage is about 16 V. That is, the Voltage Sensitive Switches must not switch at DC voltages below this 16 V level. A maximum limit to the switching voltage comes from "off hook service request" conditions. Such a value is typically 37.0 V or 18.5 V per switch. Based upon the above-mentioned minimum and maximum voltage limits, the nominal switch voltage is 17.3 V with 7% tolerance.
Further, to service certain telephone features (i.e., retaining stored numbers) a minimum level of direct current must be passed by the Voltage Sensitive Switches at voltages below the minimum switching voltage. This current is normally provided by a resistive shunt path. A resistive path of several megaohms is needed to supply sufficient current in the "on-hook idle state" condition.
In practice, there is only a limited range of current available to switch the voltage sensitive switches, and this current is affected by variations in temperature. At -40.degree. C. the maximum value is about 75 microamperes and the minimum value is 20 microamperes, after subtracting the resistive component. The corresponding values at +70.degree. C. are 50 microamperes and 2 microamperes respectively. That is, temperature greatly effects current available to cause switching.
Once the Voltage Sensitive Switch has switched into the "on-state", the "on-state" voltage drop must not exceed certain maximum values. Such values are typically 1 V at 20 mA, 1.25 V at 25 mA and 1.5 V at 30 mA. The lowest specified value of line current when the switch is in the "on-state" is 10 mA which occurs in the "return loss" condition. This 10 mA current represents the maximum value of switch holding current to maintain the switch in the "on-state" condition.
Still further, extra components are added to the MTU to facilitate line diagnostics/testing. Typically, there are added two series connected resistor and capacitor networks (see FIG. 2) which are connected between the incoming and outgoing wire to the equipment. This network and the voltage sensitive switch function create a relaxation oscillator. To ensure that the relaxation oscillator does not stop oscillating at the highest current available, the holding current must be greater than 1 mA.
In view of these conditions set forth above, the holding current must be in the range of 1 to 10 mA.
In addition to the normal service tests required, there are some stress tests involving surge and power cross that are also detailed in the standards. These state that the Voltage Sensitive Switch should be undamaged by surges of 9A for 10/360 microseconds and 6A for 10/1000 microseconds. Fusing requirements of the Voltage Sensitive Switch are also delineated and determined by AC fire hazard requirements.
In summary, there are many and varied standard specifications for the MTU with a typical circuit for such device of the prior art being shown in FIG. 2.
Referring to FIG. 2, the triggering function and the triac (Q1), resistor (R1) and capacitor (C1) form the relaxation oscillator circuit as required for line diagnostics/testing; fuse (F1) allows compliance with AC fire hazard requirements; and resistor (R2) in conjunction with Resistor (R1) provides the resistive path necessary for "on-hook idle state" condition.
A triac such as a Texas Instruments TICP206 device can be selected to have holding current, on-state voltage drop, surge and continuous AC rating all in compliance with the standards.
The major design problem with the Voltage Sensitive Switch is the solution of the triggering function. It has already been noted that the operating temperature range causes the switching voltage and current windows to be very small, thereby making the component selection for the triggering function extremely difficult without incurring high cost.
One solution to this problem is described in the U.S. Pat. No. 4,396,809, and is a discrete component solution for the triggering function. This is shown generally in FIG. 3.
An unidirectional trigger circuit is formed by resistors (R1) and (R2), transistors (Q1) and (Q2) and reference zener diode (Z1). The triggering voltage for this circuit will be given by the sum of the zener voltage and Vbe (base-emitter voltage) of transistor (Q1). Resistor (R1) controls the triggering current and resistor (R2) removes false triggering due to leakage currents. At currents above the triggering current, both transistors (Q1) and (Q2) become active and form a regenerative pair and switch on developing a total voltage drop of about 0.8 V across the transistor pair. The diode bridge comprising diodes (D1), (D2), (D3) and (D4) allows the unidirectional circuit to function in a bidirectional manner. The inclusion of the diode bridge does not change the triggering current level, but increases the triggering and on-state voltage by the sum of the forward voltage drops of diodes (D1) and (D4) or (D2) and (D3) depending on polarity.
The resultant circuit depends on the tolerancing of two components for the triggering current and six components for the triggering voltage (four components for each polarity). In an attempt to simplify and cost reduce this circuit, there have been several proposals to replace it with available multi-layer semiconductor devices. Simple avalanche devices like back to back zener diodes are not suitable for this application as there is insufficient current available to directly trigger the triac. Some form of break back or crow-bar action is required to discharge some of the capacitor (C1) energy into the triac gate at a high enough level to cause triac triggering. For this reason, proposals have been made to use break back and crow-bar devices such as DIACS and SIDACS.
DIACS and SIDACS are not readily available in the voltage window needed for MTU operation. Such devices would be difficult to manufacture with the appropriate parameters.
Another potential problem occurs during stress testing when the triggering function has to switch the rapidly rising surge current. The discrete solution described above would need to incorporate heavier duty components to withstand the surge stress and to avoid failure which would increase the cost.
In addition, the original unidirectional switch (without diode bridge) is not suitable for antiparallel connection. Resistor (R1), in conjunction with the forward biassed reference diode, would create an excessive shunt current path when the unidirectional switch was reverse biased.
Removing resistor R1 can reduce this problem. However, the component tolerancing then becomes much more critical on triggering current. Previously, the triggering current could be defined as: Vbe(Q1)/R1. In removing (R1) the triggering current becomes Vbe(Q1)/R2hFE(Q1). The new factor of hFE(Q1) means that the design of transistor Q1 in terms of hFE is very critical to the resultant triggering current.